This invention relates generally to a focuser for controlling the focus of a telescope and, more particularly, to a temperature compensating focuser compensating for changes in ambient temperature.
Particular telescope configurations, such as the Cassegrain configuration, which include primary and secondary mirrors, are susceptible to temperature drift effect on the back focuser. It is not uncommon to experience up to a 0.010-inch temperature drift for each 0.degree. C. in temperature change. Because telescopes are used during nighttime hours when temperature changes are often extensive, it is possible for the telescope to experience significant focus drift especially during long exposure periods. It is not unusual during an observation session, for the ambient temperature to change by at least 10.degree. C. within the time span of a few hours. Applications which require exact focus, such as CCD imaging or film astrophotography, typically also frequently require long exposure periods. By way of example, a typical RGB exposure sequence can last one hour. The change in focus due to the temperature is a serious problem for these telescopes resulting in frequent focusing before exposures and bloated stars during long exposures.
Although Cassegrain-type telescopes are exceptionally sensitive to temperature drift, other telescopes would exhibit such drift to a greater or lesser extent.